Method of and apparatus for separating crystals from solutions



G. B. Ax ErAL 2,315,752 METHOD OF AND APPARATUS FOR SEPARATING .CRYSTALS FROM SOLUTIONS Filed Jan. 31, 41942 v Patented Apr. 6, 1943 UNITED STATES PATENT- OFFICE METHOD OF AND APPARATUS FR SEPA- RATING CRYSTALS 'FROM SOLUTIONS Georg Bernhard Ax and Kurt Helmut Grunewald, Finspong, Oscar Anton Wiberg, Borggard, and

Erik man, stocksund, sweden; said Ax, said Grunewald, and said Wiberg assignors to said man Application January 31, 1942, Serial'No. 429,126

A In Sweden January 9, 1941 1,1 claims. (01.624124) y y ous solutionsV being used, it vhas lsolution of the substance in the solvent is obtained. This method of concentrating a solution is often more advantageous than methods involving the evaporation of the solvent by heating the solution. The freezing-out method, however, has not as yet been put to large-scale usein the art because it is difficult, in large-scale operation, to separate the concentrated solution from the crystals without prohibitive losses and simultaneously diluting the concentrated solution to a substantial degree. The large quantities of materials to be worked up whenoperating on a technical scale necessitate simple methods for handling the materials, and for this reasona separation of the concentrated solution from the crystals by an ordinary centrifugation, for instance, is not suitable. Y

The present invention has for its object to obviate these diillculties. The invention is mainly characterized by the fact that the crystals formed through the freezing-out process, said crystals being assumed to be lighter than the solution, are

separated from the solution by causing them to rise to the surface of the solution and to form there a crystal body or mass which is freed from accompanying solution by means of a washing liquid supplied from above, the topmost layer of said crystal body being removed for instance by v scraping.

As stated, the method is based on the fact that the crystals are lighter than the solution so that they collect on the surface of the liquid where they form a homogeneous body which may attain a considerable. depth. The difference between the specic gravity of the solution and that of the\ crystals may be relatively large.. Thus for example, ln aqueous'solutions the specic gravity 40 of ice'is 0.92 and that of .a concentrated solution about 1.2. Y

The method may be applied not only to aqueous solutions but to solutions generally.l The solu-y tion should, however, have such a high concentration that the specinc gravity thereof substantially exceeds the specific gravity of the solvent in the solid state.

In order to obtain the desired result the crystals in the solution should not freeze together but remain in the form of separate crystals and the crystal -body should be permeable to washing liquid supplied from above.- By exhaustive researches and tests in largescale operation, aque. 55

been found that bodies of ice crystals possess these qualit/les. y

The invention also comprises an apparatus adapted for the carrying out of the method. Such apparatus will be described in the following, ref-v erence .being had tc the attached drawing which shows a few embodiments, in connection with which other characteristic features of the invention will be disclosed.

Fig. 1 shows diagrammatically a vertical section of a separating vessel; Fig. 2 shows diagrammatically a vertical. section of a separating vessel .containing a refrigerating unit, and Fig. 3 shows,

on a larger scale, a vertical section of the top portion of a separating'vessel provided with a scraping device. A perspective view of a scraper of the scraping device is shown in Fig; 4. Fig. 5 is a more or less diagrammatic vertical section -of an apparatus including means for cooling the washing liquid.

As shown in Fig. 1 the apparatus comprises a receptacle I which is open at the top. A mixture of crystals and concentrated solution is fed into the receptacle through the supply pipe 2. The freezing has been effected in another vessel (not shown). Thus the separating vessel is used in combination with a separate freezing-out vessel.

The crystals 2|, being lighter than the solution,

rise to the surface, the solution flowing oi through the outlet 3 (if desired being returned to the freezing-outyessel) The level of the solution is in-` dicated by the dotted line 4. As shown in Fig. 1

the level 4 lies below the upper edge of the re- The ceptacle where the scraping oi takes place. difference in height is designated by 5. As the uppermost layers of the crystal body rise above the edge or rim of the receptacle they are scraped off by means of a suitable scraping device. The level of the solution maybe observed in the sight glass 6 and is controlled .by means of valves 'l and 8 or an overflow (not shown).

By suitable devices, for instance a screen 9, it is possible to prevent crystals fromescaping together with the solution flowing off through out# 5 let 3.

When a crystal body having a thickness as represented by 5 is obtained over the level of the liquid the solution accompanying the crystals will drain off from said crystals. If the upward movement of the crystals is slow the draining-oir time `will be long, and the .crystals will be freed from liquid .to aconsiderable extent, although not completely. A quantity of solution amounting to A 116 toA M1 of the weight of the crystals will remain on the surfaces of the crystals.

In order to improve the results and to effect a complete removal of solution from the crystals l the spontaneous draining-off is combined with a washing ofthe crystal body from above. Such washing is carried out in such a manner that a solvent, e. g. water, the original solution or diluted solution or mixtures of the latter is supplied to the top surface of the crystal body. ll'he dotted lines l in Fig. l represent a shower of washing liquid being supplied. Prior to the supply of the, washing liquid it is preferably cooled,

e. g. by means of the crystals removed by scraping (see Fig.

The washing also has another purpose.' When freezing out crystals from concentrated solutions some of the solute is frozen into the crystals as inclusions. This is especially the case in the treatment of highly concentrated salt solutions, e. g. sea water, and solutions of suliates. The salts occurring as inclusions are, however, completely dissolved during the washing according to the invention.

The washing liquid'mpplied does not descend .into the concentrated solution but forms a layer The crystal body is raised by the crystals form` The top layer of the growing' ing from below. crystal body is removed by scraping oi or cutting ofi'. rIhis is eected at regulated intervals so as to obtain a suitable thickness of the layer to be scraped oft. The crystal mass removed comprises crystals and washing liquid. If the washing is wellmanaged the crystal mass scraped oil comprises substantially the entire quantity of washing liquid, so that only a small quantity thereof descends into the concentrated solution, diluting the upper layer of the latter. By controlling the level diiference 5, the quantity of crystals supplied from below the body as well as the quantity of washing liquid supplied by the showers, very satisfactory washing results may be obtained.

In' contradistinction to theold washing meth- ,ods the crystals according to the present invention rise to the surface through the liquid layer below the crystal mass passing through zones where the concentration decreases in an upward direction. l

Water or dilute sulte waste liquor may be mentioned as examplesof washingliquid to be used inthe concentration of sulte liquor by freezing-out processes. Such sulte waste liquor is obtained in large quantities in connection with the washing of sulflte vpulp in. the pulp tanks.

In the concentration of sea water the washing' is preferably carried out by means of sea water.' During the washing procedure usually a very small quantity of the washing" liquid descends into the solution being concentrated, which lis introduced through the pipe 2. In certain cases it may, however, be preferable to supply, as washing liquid, the whole quantity of the solution to be concentrated.V When this is done the freezing of solution is carried out only after said solution has passed through the crystal body.

Fig. 2 shows a device provided with a refrigerator Il for the freezing out of solvent. The refrigerator is preferably located in the lower part of the receptacle. The crystals which freeze on the surfacesof the refrigerator are removed therefrom and rise to the crystal body above.y In this embodiment the original solution may be supplied through the crystal body and/or through the pipe 2, and concentrated solution may be drawn off through a pipe or by an overflow by means of which the height of the layer being washed is controlled.

if, however, the liquid rises above the desired level it is not absolutely necessary to draw olf more solution. If the concentration is unsatisfactory the supply may be decreased in order to restore the desired level and the desired concentration. This control may obviously be carred out automatically.

The crystals may be removed by means of the reciprocating lscraping device l2 (Fig. 3). itis preferred to supply the washing liquid immediately after the scraping ofi, by means of a shower device 2li, which follows the movement of the device i2. ln this way the washing is eiected intermittently. The washing liquid may, how- Y the owlrlg on is very slow and, moreover, in-

movement is obtained by means of the chain or wire I5 guidedby the rolls I6, I1, the scraper belng connected with the chain I5 through the bar I8. 'At the two extreme ends of the scraper overturning members I9 are provided which are shown more clearly in Fig. 4. When the scraper has been drawn over the crystal body and has collected a crystal layer the members I9v at the extreme position engage below the abutments or `journals 20, vthe scraper overturning andA discharging the mass ,transported thereby as indicated by the dash-dotted lines in- Fig. 3. During the return stroke the scraper,l is kept lifted above the crystal body by tliebar I8, the latter moving along with the chain in thelowerpart of its Y thus eiected intermittently, the washing liquid acting on the crystal body during the whole interval between two successive discharge strokes of the scraper which enhances the washing eiect.

The removal by scraping may also be carried out by means of a rotating scraper, the vertical wall of the receptacle then preferably being cylindrical and the scraper arranged in such a manner that its centre of rotation coincides with the central axis of the cylinder. After'one revolution of the ,scraper one layer of the crystal mass has been scraped off. By means of a transport worm or the like the scraped-oir mass is carried towards the periphery.

the Washing liquid comprises -diluted original solution.

'5. Amethod according to claim 1, in which the washing liquid is cooled by means of crystals discharged, prior to its being supplied to the` crystals to be washed,l f

6. A method according to claim 1, in which the supply of washing liquid to thel crystals isl controlled so as not to exceed that quantity which is necessary for displacing solution accompanying the crystals, substantially all of-the Washing liquidbeing carried along with the crystals dis- The growth' o! the crystal body from below is n not always equally large over the entire surface of the said body. Ordinarily such differences as may occur are compensated by the fact that the buoyancy increases at suchplaces where the body is larger. A compensation or an adjustment of the lower surface of the crystal body may, if

necessary, also be effected by means of a rotating scraping device.

When treating-solutions of low concentration it may be economicallyadvantageous to divide the freezing-out process into two or more stages. In

such cases the washing may take place during a later stage by means of solution from a preceding stage, if necessary with the alternating use of a solvent, such as pure water. f

What we claim is:

` 1. A method ofconcentrating solutions, comprising the steps of freezing put' part4 of theA solvent to form crystals, oating said crystals upwardly in a vessel containing solution to a pointj of discharge from said vessel, washing the crystals while at said point of discharge land remov-` ing the crystals from said point of discharge, and withdrawing solution freed of said crystals fro said vessel.

2. A method according to claim 1, in which the level of the solution is kept below the point of discharge `oi the crystals so as lto obtainand maintain a body of crystals above the level 'of the solution, said body being carried by the solution and the crystals therein, solution accompanying .the crystals being partly drained ci! and partly displaced by the washing liquid supplied at said point or discharge.' 3. A method according to claim 1, in which the washing liquid comprises original solution.

4. A method according vto claim 1, in which solution. l

7. A process according to claim 1, in whichall Vof the solution to be concentrated is supplied to the crystals as washing liquid.

.8. Amethod according to claim l in which the freezing-out process is carried out in at least twg stages and the washing liquid used in one stagecomprises solution1 withdrawn from another'stage. p

l 9, An apparatus for carrying out the method defined in claim 1, comprising areceptacleopen at the top and provided with a feed conduit and a discharge conduit at its lower portion, means for supplying y washingY liquid to the crystalsv floated (upwardly through the receptacle, and movable. means for scraping off washed crystals at the upper portion of said receptacle.

`10. An apparatus forcarrying out the method defined in claim 1, comprising a receptacle open at the top and providedwith a feed conduit and a discharge conduit at its lower portion, means for supplying washing liquid to the crystals floated upwardly through the receptacle, movable means for scraping oir washed' crystals at the -upper portion of said receptacle, and a refrigeray.

tor disposed in the lower portion o! the receptacle.

11. An apparatus for carryingj out the method dened in claim 1, comprising a receptacle open at the top and provided .witha feed conduit and .follow the movement of the scraping-oi! means.

GEORG BERNHARD KURT HELMUT GRUNEWALD. OSCAR ANTON ERK OMAN. 

